1. Field of the Invention
The present invention relates to a control system for a synchronous motor, and more particularly to a highly accurate and high-performance synchronous motor drive system whose controlling mechanism is made simple by not using sensors to detect the motor""s speed and position or a motor current sensor.
2. Descriptions of Prior Art
FIG. 33 shows a method for controlling a synchronous motor that does not detect the synchronous motor""s magnetic pole position or motor current and does not use a position sensor either. This prior art uses a current sensor that detects a motor current instead of using a position sensor. This prior art is based on a synchronous motor vector control that uses a position sensor, but employs a magnetic pole position estimator and a speed estimator instead of a position sensor (hereinafter, this method is referred to as a xe2x80x9csensorless vector control methodxe2x80x9d). The configuration of this method, except for a magnetic pole position detecting section and a speed detecting section, is the same configuration as a vector control method that uses a position sensor.
In FIG. 33, there are shown a speed command generator 1 for generating a rotational speed command xcfx89r*, a motor controller 2Y, a PWM generator 3 for converting a voltage command to a PWM (pulse-width modulation) pulse, an inverter 4, a synchronous motor 5, a conversion gain 6 for converting a mechanical angle frequency to an electrical angle frequency, an Id* generator 8 for generating a d-axis current command Id*, a voltage command computing device 11 for computing dq-axis voltage commands Vdc* and vqc*, a dq inverse converter 12 for converting a dq-axis value to a three-phase alternating current value, an adder 13 for adding and subtracting signals, a speed controller 27 for adjusting Iq* so that an estimated speed value agrees with a speed command, a current controller 30 for correcting voltage commands Vdc* and Vqc* so that estimated current values Idc and Iqc agree with command values Id* and Iq* respectively, a magnetic pole position estimator 37 for estimating the magnetic pole axis of the motor, a speed estimator 38 for estimating a rotational speed of the motor, a dq coordinate transformer 39 for transforming a three-phase alternating current (AC) value to a rotary coordinate value, a direct current (DC) power source 41 for the inverter, a main-circuit section 42 of the inverter, a gate driver 43 for turning on and off semiconductor switching elements Sup to Swn of the inverter main-circuit based on a PWM pulse and a current sensor 44 for detecting a current of the motor.
Referring also to FIG. 33, a magnetic pole position estimator 37 corresponds to a magnetic pole position sensor and a speed estimator 38 corresponds to a speed sensor. In addition, as is the case with the vector controller having a position sensor, a speed controller 27 and a current controller 30 are provided to automatically make adjustments so that the speed and current agree with each command value. This is, for example, described in xe2x80x9cSensorless control of the permanent magnet synchronous motor""s position by direct estimation calculation of axis errorxe2x80x9d on pages 963 to 966 of the proceedings III, No.97 issued by the Reports of JIASC Conference 2000, Japan.
A prior art of a control method which uses neither a position sensor nor a motor current sensor, shown in FIG. 34, has been disclosed in Japanese Application Patent Laid-Open Publication No. Hei 06-153526 and No. Hei 08-19263. Referring now to FIG. 34, a motor current estimator 40 estimates and computes a motor current from a DC current I0 of an inverter and a PWM pulse shape. And the same reference numerals shown in FIG. 33 are employed for denoting the same devices.
In FIG. 34, a motor current is not directly detected, but a DC current of an inverter is detected by a current sensor 44. A motor current estimator 40 estimates a motor current from the detected value I0 of the DC current and the output pulse shape of a PWM generator 3 and then outputs the estimated value I1c to a controller 2Y. Based on the I1c, the controller 2Y performs the vector-type sensorless control, for example, in the same manner as shown in FIG. 33.
Next, operations of a motor current estimator 40 will be described with reference to FIGS. 35(a) to (e). FIGS. 35(a) to (c) illustrate shapes of a PWM pulse for each phase. Plus-side switches (Sup, Svp and Swp) are turned on when the value of each phase is xe2x80x9c1xe2x80x9d and minus-side switches (Sun, Svn and Swn) are turned on when the value of each phase is xe2x80x9c0xe2x80x9d. If there is a motor current, as shown in FIG. 35(d), the detected DC current value I0 of an inverter would appear as a waveform shown in FIG. 35(e). The waveform in FIG. 35(e) has four modes as described below:
(1) Mode 1: Sup=ON, Svp=ON,
Swp=ONxe2x86x92I0=0 
(2) Mode 2: Sup=ON, Svp=ON,
Swp=OFFxe2x86x92I0=Iu+Iv=xe2x88x92Iw 
(3) Mode 3: Sup=ON, Svp=OFF,
Swp=OFFxe2x86x92I0=Iu 
(4) Mode 4: Sup=OFF, Svp=OFF, Swp=OFFxe2x86x92I0=0
Accordingly, xe2x80x9cIuxe2x80x9d can be detected by using Mode (3) for detecting a DC current and xe2x80x9cIwxe2x80x9d can be detected by using Mode (2). xe2x80x9cIvxe2x80x9d can be calculated from Iu and Iw. Thus, it is possible to reproduce a motor current using the switching condition of the inverter main circuit and DC current values. As a result, if a motor current can be estimated, the above-mentioned sensorless vector control method would be able to be realized.
In the sensorless vector control method shown in FIG. 33, a motor must be equipped with a current sensor 44. However, there is a problem that reliability may be decreased by using a sensor 44 and a cost problem also arises because an expensive current sensor may be required to realize highly accurate control. Further, the method shown in FIG. 34 has a problem of high-frequency oscillations (ringing) of a current caused by switching operations. The possibilities of ringing occurring increase as the length of the wiring cable for the motor becomes longer, which makes it difficult to detect necessary current values. Furthermore, when a rotational frequency of the motor is low, the width of a PWM pulse becomes narrow, therefore, even though the wiring cable is shortened, the method is affected by noise and detection accuracy deteriorates.
An object of the present invention is to provide a high-performance drive system for a synchronous motor which ensures reliability without having a motor current sensor and is affected little by noise such as ringing.
A synchronous motor drive system in accordance with the present invention detects a DC current of an inverter which drives a synchronous motor, and based on the magnitude of the current, estimates torque current components that flow through the motor, and then based on the estimated value, determines the voltage which is applied to the motor, and finally estimates and computes the magnetic pole axis located inside the motor by using the estimated value of the torque current.
A synchronous motor drive system in accordance with the present invention comprises a synchronous motor, an inverter which applies alternating current to the synchronous motor, a DC power source which supplies power to the inverter, means for detecting a current supplied from the DC power source to said inverter, means for giving a rotation command to said synchronous motor, means for giving current commands Id* and Iq*, Id* on the dc-axis that is assumed to be the magnetic pole axis located inside said synchronous motor and Iq* on the qc-axis that is perpendicular to the dc-axis, and means for computing dc-qc-axis voltage commands based on said current commands Id* and Iq* and said rotation command, wherein control signals are sent to said inverter based on the voltage commands, torque current components inside said synchronous motor are estimated and computed based on the detected current value of said DC power source, and then said current command Iq* is generated based on the computed value.